1. Field of the Invention
This invention relates to machines which are used to form semiconductor devices by means of photolithographic techniques. In such systems, light of a predetermined wavelength is projected through a transparent mask (also referred to as a reticle) which has a circuit pattern formed on it. The light is then passed through a reduction lens and an image of the circuit pattern is projected and exposed onto a semiconductor wafer which is coated with photoresist. Generally, a device is formed by successively exposing different mask patterns onto a wafer in an overlapping fashion. The present invention is directed to the illumination system which is used to project light through the mask.
2. Description of the Prior Art
A typical prior art system for illuminating a mask is shown in FIG. 1. This system includes a mercury arc lamp 10 located within an aconic reflector 12. In conjuction with the reflector, the lamp provides a light output which has a Gaussian distribution of energy from the center to the edge of the illumination area (i.e., the intensity at the center of the illumination area is much greater than at the edge). The light is passed through a series of reflectors and filters, indicated generally at 14, where unwanted wavelengths are removed. The light is then passed through a light mixer subassembly 16 which includes a matrix of stick lenses (i.e., lenses in which the ratio of length to diameter is large). Photosensitive materials which are employed in the photolithographic process are very light sensitive, and it is critical that light striking the coated wafer be of relatively uniform intensity. The function of the light mixer subassembly 16 is to even out the light distribution so as to reduce intensity deviation throughout the illuminated area to less than about ten percent. Light from the mixer subassembly is passed through a combining lens 18. Light from the combining lens is then passed through a transfer lens assembly 20 and directed toward a mask 22. The mask is a transparent plate which includes a circuit pattern formed thereon, and the light from the lamp 10 illuminates the patterns and causes an image of the circuit to be projected and exposed onto a semiconductor wafer (not shown). Typically, the circuit image is projected through a reduction lens and the image exposed onto the wafer is smaller than the mask pattern by a factor of about ten.
In order to define the area of the mask 22 which is to be illuminated, an assembly known as a reticle edge masking assembly 24, or REMA, is positioned adjacent the mask 22. The REMA includes four blades 24a-d which are orthogonally positioned and are movable in order to define the size of a rectangular opening 26. Opposing blades are driven with a motor and a series of pulleys (not shown) so that they move at a constant rate.
The REMA assembly of the prior art has several disadvantages associated with it. Primary among these is the problem of near field diffraction or Fresnel effects. The mask 22 is located in a focal plane of the lens assembly of the illumination system, and light from the individual stick lenses of the mixer subassembly 16 will be focused at the mask plane. Light striking the REMA will be diffracted somewhat, thereby resulting in blurring at the edges of the illuminated area. This problem can be substantially overcome by locating the REMA extremely close to the mask plane; however, the physical constraints of the system are such that it is not possible to locate the REMA as close to the mask plane as desired. Because the REMA must be located as close as possible to the mask plane, the structure of the assembly must be relatively simple. In certain applications it may be desirable to produce illumination patterns other than a rectangle centered with the circuit pattern (e.g., a circular or rectangular illumination configuration which illuminates only a quadrant of the mask pattern may be desired). The structural limitations of prior art systems effectively prohibit the design of a REMA with such multiple illumination patterns.
Thus, the prior art REMA assembly is located as close as possible to the mask plane and operates by silhouetting the illumination which is projected toward the mask plane. This type of operation results in edge blurriness caused by near field diffraction. As manufacturing techniques improve, the fineness of the illumination pattern projected onto the mask becomes critical. Present systems operate with micron tolerances, and the blurring effects at the edges of the illumination patterns can adversely effect the operation of a system.